Distinct long chain and very long chain fatty acyl CoA synthetases in rat liver peroxisomes and microsomes

FATP2-targeted therapies - A role beyond fatty liver disease

Fatty acid transport protein 2 (FATP2) is a multifunctional protein whose specific function is determined by the type of located cell, its intracellular location, or organelle-specific interactions. In the different diseases setting, a newfound appreciation for the biological function of FATP2 has come into view. Two main functions of FATP2 are to activate long-chain fatty acids (LCFAs) as a very long-chain acyl-coenzyme A (CoA) synthetase (ACSVL) and to transport LCFAs as a fatty acid transporter. FATP2 is not only involved in the occurrence of nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM), but also plays an important role in lithogenic diet-induced cholelithiasis, the formation of cancer tumor immunity, the progression of chronic kidney disease (CKD), and the regulation of zoledronate-induced nephrotoxicity. Herein, we review the updated information on the role of FATP2 in related diseases. In particular, we discuss the new functions of FATP2 and propose that FATP2 is a potential clinical biomarker and therapeutic target. In conclusion, regulatory strategies for FATP2 may bring new treatment options for cancer and lipid metabolism-related disorders.

Rapid affinity purification of intracellular organelles using a twin strep tag

Cells are internally organized into compartmentalized organelles that execute specialized functions. To understand the functions of individual organelles and their regulations, it is critical to resolve the compositions of individual organelles, which relies on a rapid and efficient isolation method for specific organellar populations. Here, we introduce a robust affinity purification method for rapid isolation of intracellular organelles (e.g. lysosomes, mitochondria and peroxisomes) by taking advantage of the extraordinarily high affinity between the twin strep tag and streptavidin variants. With this method, we can isolate desired organelles with high purity and yield in 3 min from the post-nuclear supernatant of mammalian cells or less than 8 min for the whole purification process. Using lysosomes as an example, we show that the rapid procedure is especially useful for studying transient and fast cellular activities, such as organelle-initiated signaling and organellar contents of small-molecular metabolites. Therefore, our method offers a powerful tool to dissect spatiotemporal regulation and functions of intracellular organelles.

Mouse very long-chain acyl-CoA synthetase in X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is a neurodegenerative disorder characterized by accumulation of very long-chain fatty acids (VLCFA). This accumulation has been attributed to decreased VLCFA beta-oxidation and peroxisomal very long-chain acyl-CoA synthetase (VLCS) activity. The X-ALD gene, ABCD1, encodes a peroxisomal membrane ATP binding cassette transporter, ALDP, that is hypothesized to affect VLCS activity in peroxisomes by direct interaction with the VLCS enzyme. Recently, a VLCS gene that encodes a protein with significant sequence identity to known rat and human peroxisomal VLCS protein has been identified in mice. We find that the mouse VLCS gene (Vlcs) encodes an enzyme (Vlcs) with VLCS activity that localizes to peroxisomes and is expressed in X-ALD target tissues. We show that the expression of Vlcs in the peroxisomes of X-ALD mouse fibroblasts improves VLCFA beta-oxidation in these cells, implying a role for this enzyme in the biochemical abnormality of X-ALD. X-ALD mice, which accumulate VLCFA in tissues, show no change in the expression of Vlcs, the subcellular localization of Vlcs, or general peroxisomal VLCS activity. These observations imply that ALDP is not necessary for the proper expression or localization of Vlcs protein, and the control of VLCFA levels does not depend on the direct interaction of Vlcs and ALDP.

Synthesis of dinucleoside polyphosphates catalyzed by firefly luciferase and several ligases

The findings presented here originally arose from the suggestion that the synthesis of dinucleoside polyphosphates (Np(n)N) may be a general process involving enzyme ligases catalyzing the transfer of a nucleotidyl moiety via nucleotidyl-containing intermediates, with release of pyrophosphate. Within this context, the characteristics of the following enzymes are presented. Firefly luciferase (EC 1.12. 13.7), an oxidoreductase with characteristics of a ligase, synthesizes a variety of (di)nucleoside polyphosphates with four or more inner phosphates. The discrepancy between the kinetics of light production and that of Np(n)N synthesis led to the finding that E*L-AMP (L = dehydroluciferin), formed from the E*LH(2)-AMP complex (LH(2) = luciferin) shortly after the onset of the reaction, was the main intermediate in the synthesis of (di)nucleoside polyphosphates. Acetyl-CoA synthetase (EC 6.2.1.1) and acyl-CoA synthetase (EC 6.2.1. 8) are ligases that synthesize p(4)A from ATP and P(3) and, to a lesser extent, Np(n)N. T4 DNA ligase (EC 6.5.1.1) and T4 RNA ligase (EC 6.5.1.3) catalyze the synthesis of Np(n)N through the formation of an E-AMP complex with liberation of pyrophosphate. DNA is an inhibitor of the synthesis of Np(n)N and conversely, P(3) or nucleoside triphosphates inhibit the ligation of a single-strand break in duplex DNA catalyzed by T4 DNA ligase, which could have therapeutic implications. The synthesis of Np(n)N catalyzed by T4 RNA ligase is inhibited by nucleoside 3'(2'),5'-bisphosphates. Reverse transcriptase (EC 2.7.7.49), although not a ligase, catalyzes, as reported by others, the synthesis of Np(n)ddN in the process of removing a chain termination residue at the 3'-OH end of a growing DNA chain.

Role and organization of peroxisomal beta-oxidation

In mammals, peroxisomes are involved in breakdown of very long chain fatty acids, prostanoids, pristanic acid, dicarboxylic fatty acids, certain xenobiotics and bile acid intermediates. Substrate spectrum and specificity studies of the four different beta-oxidation steps in rat and/or in man demonstrate that these substrates are degraded by separate beta-oxidation systems composed of different enzymes. In both species, the enzymes acting on straight chain fatty acids are palmitoyl-CoA oxidase, an L-specific multifunctional protein (MFP-1) and a dimeric thiolase. In liver, bile acid intermediates undergo one cycle of beta-oxidation catalyzed by trihydroxycoprostanoyl-CoA oxidase (in rat), or branched chain acyl-CoA oxidase (in man), a D-specific multifunctional protein (MFP-2) and SCPX-thiolase. Finally, pristanic acid is degraded in rat tissues by pristanoyl-CoA oxidase, the D-specific multifunctional protein-2 and SCPX-thiolase. Although in man a pristanoyl-CoA oxidase gene is present, so far its product has not been found. Hence, pristanoyl-CoA is believed to be desaturated in human tissues by the branched chain acyl-CoA oxidase. Due to the stereospecificity of the oxidases acting on 2-methyl-branched substrates, an additional enzyme, 2-methylacyl-CoA racemase, is required for the degradation of pristanic acid and the formation of bile acids.

Intraperoxisomal localization of very-long-chain fatty acyl-CoA synthetase: implication in X-adrenoleukodystrophy

X-adrenoleukodystrophy (X-ALD) is a demyelinating disorder characterized by the accumulation of saturated very-long-chain (VLC) fatty acids (>C(22:0)) due to the impaired activity of VLC acyl-CoA synthetase (VLCAS). The gene responsible for X-ALD was found to code for a peroxisomal integral membrane protein (ALDP) that belongs to the ATP binding cassette superfamily of transporters. To understand the function of ALDP and how ALDP and VLCAS interrelate in the peroxisomal beta-oxidation of VLC fatty acids we investigated the peroxisomal topology of VLCAS protein. Antibodies raised against a peptide toward the C-terminus of VLCAS as well as against the N-terminus were used to define the intraperoxisomal localization and orientation of VLCAS in peroxisomes. Indirect immunofluorescent and electron microscopic studies show that peroxisomal VLCAS is localized on the matrix side. This finding was supported by protease protection assays and Western blot analysis of isolated peroxisomes. To further address the membrane topology of VLCAS, Western blot analysis of total membranes or integral membranes prepared from microsomes and peroxisomes indicates that VLCAS is a peripheral membrane-associated protein in peroxisomes, but an integral membrane in microsomes. Moreover, peroxisomes isolated from cultured skin fibroblasts from X-ALD patients with a mutation as well as a deletion in ALDP showed a normal amount of VLCAS. The consequence of VLCAS being localized to the luminal side of peroxisomes suggests that ALDP may be involved in stabilizing VLCAS activity, possibly through protein-protein interactions, and that loss or alterations in these interactions may account for the observed loss of peroxisomal VLCAS activity in X-ALD.

Metabolism of trideuterated iso-lignoceric acid in rats in vivo and in human fibroblasts in culture

Saturated very long chain fatty acids (fatty acids with greater than 22 carbon atoms; VLCFA) accumulate in peroxisomal disorders, but there is little information on their turnover in patients. To determine the suitability of using stable isotope-labeled VLCFA in patients with these disorders, the metabolism of 22-methyl[23,23,23-2H3]tricosanoic (iso-lignoceric) acid was studied in rats in vivo and in human skin fibroblasts in culture. The deuterated iso-VLCFA was degraded to the corresponding 16- and 18-carbon iso-fatty acids by rats in vivo and by normal human skin fibroblasts in culture, but there was little or no degradation in peroxisome-deficient (Zellweger's syndrome) fibroblasts, indicating that its oxidation was peroxisomal. Neither the 14-, 20-, and 22-carbon iso-fatty acids nor the corresponding odd-chain metabolites could be detected. In the rat, the organ containing most of the iso-lignoceric acid, and its breakdown products, was the liver, whereas negligible amounts were detected in the brain, suggesting that little of the fatty acid crossed the blood-brain barrier. Our data indicate that VLCFA labeled with deuterium at the omega-position of the carbon chain are suitable derivatives for the in vivo investigation of patients with defects in peroxisomal beta-oxidation because they are metabolized by the same pathways as the corresponding n-VLCFA. Moreover, as iso-VLCFA and their beta-oxidation products are readily separated from the corresponding n-fatty acids by normal chromatographic procedures, the turnover of VLCFA can be more precisely measured.

Human liver-specific very-long-chain acyl-coenzyme A synthetase: cDNA cloning and characterization of a second enzymatically active protein

Activation of fatty acids, catalyzed by acyl-coenzyme A (acyl-CoA) synthetases, is required for their subsequent metabolism. Peroxisomes and microsomes contain very-long-chain acyl-CoA synthetases (VLCSs) capable of activating fatty acids with a chain length of 22 or more carbons. Decreased peroxisomal VLCS activity is, in part, responsible for the biochemical pathology in X-linked adrenoleukodystrophy (X-ALD), illustrating the importance of VLCSs in cellular fatty acid homeostasis. We previously cloned two human genes encoding proteins homologous to rat peroxisomal VLCS; one (hVLCS) is the human ortholog to the rat VLCS gene and another (hVLCS-H1) encodes a related heart-specific protein. Here, we report the cloning of a third gene (hVLCS-H2) and characterization of its protein product. The hVLCS-H2 gene is located on human chromosome 19 and encodes a 690-amino-acid protein. The amino acid sequence of hVLCS-H2 is 44-45% identical and 67-69% similar to those of both hVLCS and hVLCS-H1. COS-1 cells transiently overexpressing hVLCS-H2 activated the very-long-chain fatty acid lignocerate (C24:0) at a rate >1.5-fold higher than that of nontransfected cells (P < 0.002). The hVLCS-H2-dependent activation of long- and branched-chain fatty acids following transient transfection was less striking. However, hVLCS-H2-dependent acyl-CoA synthetase activity with long- and very-long-chain fatty acid substrates was detected in COS-1 cells stably expressing hVLCS-H2. For all substrates tested (C18:0, C20:0, C24:0, C26:0), the hVLCS-H2 catalyzed activity was significantly increased (P < 0.01 to P < 0.0001). By both Northern analysis and reverse transcription polymerase chain reaction, hVLCS-H2 is expressed primarily in liver. Indirect immunofluorescence of COS-1 cells or human hepatoma-derived HepG2 cells expressing epitope-tagged hVLCS-H2 revealed that the protein was associated with the endoplasmic reticulum but not with peroxisomes. Thus, the primary role of hVLCS-H2 is likely to be in fatty acid elongation or complex lipid synthesis rather than in degradation.

The molecular basis of malonyl-CoA decarboxylase deficiency

We characterized a 2.1-kb human cDNA with a 1362-bp (454-amino acid) open reading frame showing 70.3% amino acid identity to goose malonyl-CoA decarboxylase (MCD). We have identified two different homozygous mutations in human MCD (hMCD) by using RT-PCR analysis of fibroblast RNA from two previously reported consanguineous Scottish patients with MCD deficiency. The first mutation is a 442C-->G transversion resulting in a premature stop codon (S148X) in the N-terminal half of the protein. The second is a 13-bp insertion in the mature RNA, causing a frameshift with predicted protein truncation. This insertion is the result of an intronic mutation generating a novel splice acceptor sequence (IVS4-14A-->G). Both mutations were found to segregate appropriately within the families and were not found in 100 normal unrelated individuals. These mutations would be predicted to cause MCD deficiency, thus confirming this transcript as the hMCD ortholog. The peptide sequence of hMCD revealed a C-terminal peroxisomal targeting sequence (-SKL). This targeting signal appears to be functional in vivo, since the distribution of MCD enzymatic activity in rat liver homogenates-as measured by means of subcellular fractionation-strongly suggests that MCD is localized to peroxisomes in addition to the mitochondrial localization reported elsewhere. These data strongly support this cDNA as encoding human MCD, an important regulator of fatty acid metabolism.

Human very long-chain acyl-CoA synthetase and two human homologs: initial characterization and relationship to fatty acid transport protein

Several human genes with a high degree of homology to rat very long-chain acyl-CoA synthetase (rVLCS) and mouse fatty acid transport protein (mFATP) were identified. Full-length cDNA clones were obtained for three genes, and predicted amino acid sequences were generated. Initial characterization indicated that one gene was most likely hVLCS, the human ortholog of rVLCS. The other two (hVLCS-H1 and hVLCS-H2) were more closely related to rVLCS than to mFATP. Phylogenetic analysis of amino acid sequences confirmed that hVLCS-H1 and hVLCS-H2 were evolutionarily closer to VLCSs than FATPs. Alignment of predicted amino acid sequences of human, rat and mouse VLCSs and FATPs revealed the existence of two highly conserved motifs. While one motif is also present in long-chain acyl-CoA synthetases, the other serves to distinguish the VLCS/FATP family from the long-chain synthetase family. Elucidation of the biochemical functions of all VLCS/FATP family members should provide new insights into cellular fatty acid metabolism.

Human very-long-chain acyl-CoA synthetase: cloning, topography, and relevance to branched-chain fatty acid metabolism

Very-long-chain acyl-CoA synthetases (VLCS) activate very-long-chain fatty acids (VLCFA) containing 22 or more carbons to their CoA derivatives. We cloned the human ortholog (hVLCS) of the gene encoding the rat liver enzyme (rVLCS). Both hVLCS and rVLCS contain 620 amino acids, are expressed primarily in liver and kidney, and have a potential peroxisome targeting signal 1 (-LKL) at their carboxy termini. When expressed in COS-1 cells, hVLCS activated the VLCFA lignoceric acid (C24:0), a long-chain fatty acid (C16:0), and two branched-chain fatty acids, phytanic acid and pristanic acid. Immunofluorescence and immunoblot studies localized hVLCS to both peroxisomes and endoplasmic reticulum. In peroxisomes of HepG2 cells, hVLCS was topographically oriented facing the matrix and not the cytoplasm. This orientation, coupled with the observation that hVLCS activates branched-chain fatty acids, suggests that hVLCS could play a role in the intraperoxisomal reactivation of pristanic acid produced via alpha-oxidation of phytanic acid.

Acyl-CoA synthetase catalyzes the synthesis of diadenosine hexaphosphate (Ap6A)

The synthesis of diadenosine hexaphosphate (Ap6A), a potent vasoconstrictor, is catalyzed by acyl-CoA synthetase from Pseudomonas fragi. In a first step AMP is transferred from ATP to tetrapolyphosphate (P4) originating adenosine pentaphosphate (p5A) which, subsequently, is the acceptor of another AMP moiety from ATP generating diadenosine hexaphosphate (Ap6A). Diadenosine pentaphosphate (Ap5A) and diadenosine tetraphosphate (Ap4A) were also synthesized in the course of the reaction. In view of the variety of biological effects described for these compounds the potential capacity of synthesis of diadenosine polyphosphates by the mammalian acyl-CoA synthetases may be relevant.

Disruption of the Saccharomyces cerevisiae FAT1 gene decreases very long-chain fatty acyl-CoA synthetase activity and elevates intracellular very long-chain fatty acid concentrations

Activation of fatty acids to their coenzyme A derivatives is necessary for subsequent metabolism. Very long-chain fatty acids, which accumulate in tissues of patients with X-linked adrenoleukodystrophy, are activated by very long-chain acyl-CoA synthetase (VLCS) normally found in peroxisomes and microsomes. We identified a candidate yeast VLCS gene (FAT1), previously identified as encoding a fatty acid transport protein, by its homology to rat liver peroxisomal VLCS. Disruption of this gene decreased, but did not abolish, cellular VLCS activity. Fractionation studies showed that VLCS activity, but not long-chain acyl-CoA synthetase activity, was reduced to about 40% of wild-type level in both 27,000 x g supernatant and pellet fractions. Separation of organelles in the pellet fraction by density gradient centrifugation revealed that VLCS activity was associated with peroxisomes and microsomes but not mitochondria. FAT1 deletion strains exhibited decreased growth on medium containing dextrose, oleic acid, and cerulenin, an inhibitor of fatty acid synthesis. FAT1 deletion strains grown on either dextrose or oleic acid medium accumulated very long-chain fatty acids. Compared with wild-type yeast, C22:0, C24:0, and C26:0 levels were increased approximately 20-, 18-, and 3-fold in deletion strains grown on dextrose, and 2-, 7-, and 5-fold in deletion strains grown on oleate. Long-chain fatty acid levels in wild-type and deletion strains were not significantly different. All biochemical defects in FAT1 deletion strains were restored to normal after functional complementation with the FAT1 gene. The level of VLCS activity measured in both wild-type and deletion yeast strains transformed with FAT1 cDNA paralleled the level of expression of the transgene. The extent of both the decrease in peroxisomal VLCS activity and the very long-chain fatty acid accumulation in the yeast FAT1 deletion model resembles that observed in cells from X-linked adrenoleukodystrophy patients. These studies suggest that the FAT1 gene product has VLCS activity that is essential for normal cellular very long-chain fatty acid homeostasis.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Sequestration of coenzyme A by the industrial surfactant, Toximul MP8. A possible role in the inhibition of fatty-acid beta-oxidation in a surfactant/influenza B virus mouse model for acute hepatic encephalopathy

We have investigated the mechanistic basis of our recent observation that exposing young mice to an industrial surfactant potentiates the inhibition of fatty-acid beta-oxidation that occurs with subsequent virus infection (Murphy et al., Biochim. Biophys. Acta 1315, 208-216, 1996). In our mouse model for acute hepatic encephalopathy (AHE), neonatal mice were painted on their abdomens from birth to postnatal day 12 with nontoxic amounts of the industrial surfactant, Toximul MP8 (Tox), and then infected with a sublethal dose (LD30) of mouse-adapted human Influenza B (Lee) virus (FluB). Mortality in mice treated with Tox + FluB was significantly higher than that in mice treated with FluB alone. In vitro assays of hepatic beta-oxidation of [1-(14)C]palmitic and [1-(14)C]octanoic acids in the presence or absence of exogenous coenzyme A (CoA) indicated that Tox-mediated inhibition of oxidation was masked when CoA was added to the assays. FluB also inhibited beta-oxidation by 20-30%, however this effect was independent of exogenous CoA which suggested that it involved a different mechanism. Tox-mediated potentiation of the inhibitory effect was most obvious (> 80% inhibition) when assays were done without added CoA. Analysis of hepatic CoA and its esters indicated that levels of both free CoA and acetyl-CoA were significantly lower in mice that were painted with Tox for 12 days. Tox-dependent reductions of acetyl-CoA were transient and returned to normal values after cessation of painting, whereas those of CoA persisted. FluB infection alone significantly reduced hepatic acetyl-CoA and the magnitude of this reduction (> 30%) was not affected by pre-exposing the mice to Tox. Relative to control mice, levels of acid insoluble acyl-CoA esters were elevated significantly in FluB and Tox + FluB treated mice. Activation of both [1-(14)C]palmitic and [1-(14)C]octanoic acids was reduced in Tox-exposed mice at experimental day 12, but only when exogenous CoA was not included in the assay media; this effect appeared to persist after cessation of painting. Collectively, these data support the concept that Tox and FluB have independent effects on hepatic CoA metabolism that are associated with abnormalities in fatty-acid beta-oxidation. However, these do not fully explain the synergistic effect of the virus and chemical on beta-oxidation inhibition, which is a candidate co-mechanism for potentiation of mortality in this mouse model of AHE.

Topology of carnitine palmitoyltransferase I in the mitochondrial outer membrane

The topology of carnitine palmitoyltransferase I (CPT I) in the outer membrane of rat liver mitochondria was studied using several approaches. 1. The accessibility of the active site and malonyl-CoA-binding site of the enzyme from the cytosolic aspect of the membrane was investigated using preparations of octanoyl-CoA and malonyl-CoA immobilized on to agarose beads to render them impermeant through the outer membrane. Both immobilized ligands were fully able to interact effectively with CPT I. 2. The effects of proteinase K and trypsin on the activity and malonyl-CoA sensitivity of CPT I were studied using preparations of mitochondria that were either intact or had their outer membranes ruptured by hypo-osmotic swelling (OMRM). Proteinase K had a marked but similar effect on CPT I activity irrespective of whether only the cytosolic or both sides of the membrane were exposed to it. However, it affected sensitivity more rapidly in OMRM. By contrast, trypsin only reduced CPT I activity when incubated with OMRM. The sensitivity of the residual CPT I activity was unaffected by trypsin. 3. The proteolytic fragments generated by these treatments were studied by Western blotting using three anti-peptide antibodies raised against linear epitopes of CPT I. These showed that a proteinase K-sensitive site close to the N-terminus was accessible from the cytosolic side of the membrane. No trypsin-sensitive sites were accessible in intact mitochondria. In OMRM, both proteinase K and trypsin acted from the inter-membrane space side of the membrane. 4. The ability of intact mitochondria and OMRM to bind to each of the three anti-peptide antibodies was used to study the accessibility of the respective epitopes on the cytosolic and inter-membrane space sides of the membrane. 5. The results of all these approaches indicate that CPT I adopts a bitopic topology within the mitochondrial outer membrane; it has two transmembrane domains, and both the N- and C-termini are exposed on the cytosolic side of the membrane, whereas the linker region between the transmembrane domains protrudes into the intermembrane space.

Biochemistry of peroxisomes in health and disease

The ubiquitous distribution of peroxisomes and the identification of a number of inherited diseases associated with peroxisomal dysfunction indicate that peroxisomes play an essential part in cellular metabolism. Some of the most important metabolic functions of peroxisomes include the synthesis of plasmalogens, bile acids, cholesterol and dolichol, and the oxidation of fatty acids (very long chain fatty acids > C22, branched chain fatty acids (e.g. phytanic acid), dicarboxylic acids, unsaturated fatty acids, prostaglandins, pipecolic acid and glutaric acid). Peroxisomes are also responsible for the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen species (e.g. O-2 and H2O2). Peroxisomal diseases result from the dysfunction of one or more peroxisomal metabolic functions, the majority of which manifest as neurological abnormalities. The quantitation of peroxisomal metabolic functions (e.g. levels of specific metabolites and/or enzyme activity) has become the basis of clinical diagnosis of diseases associated with the organelle. The study of peroxisomal diseases has also contributed towards the further elucidation of a number of metabolic functions of peroxisomes.

Molecular cloning of cDNA encoding rat very long-chain acyl-CoA synthetase

The cDNA encoding rat very long-chain acyl-CoA synthetase (VLACS) was cloned, using degenerative primers synthesized according to the partial amino acid sequences of the peptide fragments of the purified rat liver enzyme. The longest cDNA insert was 2972 base pairs with a 1860-base pair open reading frame encoding 620 amino acids. The calculated molecular mass of 70,692 daltons was consistent with size of the purified enzyme. In Northern blot analysis, a single band was detected at the position of about 3 kilobases, corresponding to the size of the cloned cDNA. cDNA-directed expression in Escherichia coli resulted in accumulation of expressed protein, as an inclusion body. An antibody was raised using this expressed protein to characterize the cDNA and the enzyme. The subcellular localization of VLACS in peroxisomes and microsomes was demonstrated in Western blot analysis. The specific activity and the substrate specificity of the cDNA expressed enzyme in COS-1 cells were consistent with those of the purified rat enzyme. The predicted amino acid sequence of VLACS had a high sequence similarity to fatty acid transport protein (Schaffer, J. E., and Lodish, H. F. (1994) Cell 79, 427-436), and was considered to have domains for adenylation and thioester formation. The entire structure of VLACS was dissimilar to that of long-chain acyl-CoA synthetase (Suzuki, H., Kawarabayashi, Y., Kondo, Y., Abe, T., Nishikawa, K., Kimura, S., Hashimoto, T., and Yamamoto, T. (1990) J. Biol. Chem. 265, 8681-8685), except for the domains.

Natural anthracenone subcellular distribution and effects on NADPH-cytochrome P450 reductase microsomal activity

Natural anthracenone subcellular distribution and effects on NADPH-cytochrome P450 reductase microsomal activity. Subcellular distribution study of a natural anthracenone (T-514) isolated from Karwinskia humboldtiana showed to be homogeneous on subcellular (nuclear, mitochondrial, peroxisomal and microsomal) fractions prepared from rat liver treated with an acute dose of T-514. These results indicate that T-514 can pass easily through subcellular compartment membranes and an absence of selectivity for some subcellular organelles. A significant increase of protein on liver homogenates and NADPH-cytochrome P450 reductase microsomal activity indicates that T-514 may act as a microsomal enzymatic inducer. In addition, this enzymatic specific activity increment could be due to the interaction of T-514 with the microsomal redox cycling.

Eicosapentaenoic acid, but not docosahexaenoic acid, increases mitochondrial fatty acid oxidation and upregulates 2,4-dienoyl-CoA reductase gene expression in rats

The aim of the present study was to investigate whether eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) was responsible for the triglyceride-lowering effect of fish oil. In rats fed a single dose of EPA as ethyl ester (EPA-EE), the plasma concentration of triglycerides was decreased at 8 h after acute administration. This was accompanied by an increased hepatic fatty acid oxidation and mitochondrial 2,4-dienoyl-CoA reductase activity. The steady-state level of 2,4-dienoyl-CoA reductase mRNA increased in parallel with the enzyme activity. An increased hepatic long-chain acyl-CoA content, but a reduced amount of hepatic malonyl-CoA, was obtained at 8 h after acute EPA-EE treatment. On EPA-EE supplementation, both EPA (20:5n-3) and docosapentaenoic acid (DPA, 22:5n-3) increased in the liver, whereas the hepatic DHA (22:6n-3) concentration was unchanged. On DHA-EE supplementation retroconversion to EPA occurred. No statistically significant differences were found, however, for mitochondrial enzyme activities, malonyl-CoA, long-chain acyl-CoA, plasma lipid levels, and the amount of cellular fatty acids between DHA-EE treated rats and their controls at any time point studied. In cultured rat hepatocytes, the oxidation of [1-14C]palmitic acid was reduced by DHA, whereas it was stimulated by EPA. In the in vivo studies, the activities of phosphatidate phosphohydrolase and acetyl-CoA carboxylase were unaffected after acute EPA-EE and DHA-EE administration, but the fatty acyl-CoA oxidase, the rate-limiting enzyme in peroxisomal fatty acid oxidation, was increased after feeding these n-3 fatty acids. The hypocholesterolemic properties of EPA-EE may be due to decreased 3-hydroxy-3-methylglutaryl-CoA reductase activity. Furthermore, replacement of the ordinary fatty acids, i.e., the monoenes (16:1n-7, 18:1n-7, and 18:1n-9) with EPA and some conversion to DPA concomitant with increased fatty acid oxidation is probably the mechanism leading to changed fatty acid composition. In contrast, DHA does not stimulate fatty acid oxidation and, consequently, no such displacement mechanism operates. In conclusion, we have obtained evidence that EPA, and not DHA, is the fatty acid primarily responsible for the triglyceride-lowering effect of fish oil in rats.

Isolation from bovine liver mitochondria and characterization of three distinct carboxylic acid: CoA ligases with activity toward xenobiotics

A mitochondrial freeze/thaw lysate was fractionated on a DEAE-cellulose column into four distinct acyl-CoA ligase fractions. First to elute was a 50 kDa short-chain ligase that activated only short-chain fatty acids. Next to elute were three ligases that had activity toward both medium-chain fatty acids and xenobiotic carboxylic acids; these were termed xenobiotic/medium-chain ligases (X-ligases) and labeled XL-I, XL-II, and XL-III, respectively, based on order of elution. The molecular weight of X-ligases I, II, and III were ca. 55,000, 55,500 and 53,000, respectively. Form XL-III showed no pH optimum; the rate increased steadily with pH beginning from pH 7.0. XL-I and XL-II showed the same behavior with benzoate as substrate, but with medium-chain fatty acids, both forms had a pH optimum at 8.8. The three X-ligases differed in substrate specificity. XL-I was the predominant nicotinic acid activating form and had the lowest Km for benzoate. Form XL-II was the only form with measurable salicylate activity, although it was extremely low. XL-III was the only 2,4,6,8-decatetraenoic acid activating form and also was the predominant medium-chain fatty acid-activating form. By comparison of substrate specificities, it was concluded that the two previously reported ligase preparations were mixtures of the three forms. When the ligase rates were compared to previously determined N-acyltransferase rates toward benzoyl-CoA and phenylacetyl-CoA, the data showed that ligase activities are 100-fold lower, and thus the ligase is rate limiting for the conjugation of both of these xenobiotics.

EPR study of annexin V-cardiolipin Ca-mediated interaction in phospholipid vesicles and isolated mitochondria

The properties of the binding of annexin V to variously composed phospholipid vesicles have been studied by applying a recently developed EPR method, using an annexin V spin label. By this approach, this protein is seen to bind to acidic phospholipid-containing vesicles, as reported, thus confirming the reliability of the method. In addition, binding of this annexin to cardiolipin-containing vesicles has been studied in more depth, and the protein has been shown to have a distinct affinity for this phospholipid. As a cardiolipin-rich natural membrane system, mitochondrial membranes and mitoplasts from rat liver were considered, and a strong binding of AV to these membranes was observed. Having compared this binding with that to phospholipid vesicles, cardiolipin-rich microdomains in the mitochondrial membranes are proposed as the putative mitochondrial binding sites for annexin V.

Substrate specificity of rat liver mitochondrial carnitine palmitoyl transferase I: evidence against alpha-oxidation of phytanic acid in rat liver mitochondria

The two branched chain fatty acids pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) were converted to co-enzyme A thioesters by rat liver mitochondrial outer membranes. However, these branched chain fatty acids could not be converted to pristanoyl and phytanoyl carnitines, respectively, by mitochondrial outer membranes. As expected, the unbranched long chain fatty acids, stearic acid and palmitic acid, were rapidly converted to stearoyl and palmitoyl carnitines, respectively, by mitochondrial outer membranes. These observations indicate that the branched chain fatty acids could not be transported into mitochondria. The data presented strongly suggest that in rat liver, alpha-oxidation of phytanic acid occurs in organelles other than mitochondria.

Very long chain fatty acids in higher animals--a review

Fatty acids with greater than 22 carbon atoms (very long chain fatty acids, VLCFA) are present in small amounts in most animal tissues. Saturated and monoenoic VLCFA are major components of brain, while the polyenoic VLCFA occur in significant amounts in certain specialized animal tissues such as retina and spermatozoa. Biosynthesis of VLCFA occurs by carbon chain elongation of shorter chain fatty acid precursors while beta-oxidation takes place almost exclusively in peroxisomes. Mitochondria are unable to oxidize VLCFA because they lack a specific VLCFA coenzyme A synthetase, the first enzyme in the beta-oxidation pathway. VLCFA accumulate in the tissues of patients with inherited abnormalities in peroxisomal assembly, and also in individuals with defects in enzymes catalyzing individual reactions along the beta-oxidation pathway. It is believed that the accumulation of VLCFA in patient tissues contributes to the severe pathological changes which are a feature of these conditions. However, little is known of the role of VLCFA in normal cellular processes, and of the molecular basis for their contribution to the disease process. The present review provides an outline of the current knowledge of VLCFA including their biosynthesis, degradation, possible function and involvement in human disease.

Acyl-CoA synthetase activity depends on the phospholipid composition of rat liver plasma membranes

The dependence of acyl-CoA synthetase on the lipid composition of rat liver plasma membranes has been investigated. For this purpose the composition of the membranes was modified by incorporation of different phospholipids in the presence of partially purified lipid transfer proteins. Another approach to the modification of the membrane phospholipid composition was treatment with exogenous phospholipase C and subsequent enrichment with different phospholipids. The experiments performed in vitro indicated that the presence of certain phospholipids such as phosphatidylnositol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylserine was essential for the activation of long chain fatty acids by acyl-CoA synthetase. However, some differences were observed when oleate and palmitate were used as substrates. Sphingomyelin was found to inhibit this activity especially when oleic acid served as substrate. In addition, we tried to modify in vivo the membrane lipid composition by treatment with D-galactosamine, which is known to induce acute hepatitis and cause biochemical and biophysical alterations in liver membranes. The results thus obtained confirmed the idea that the augmentation of the membrane lipids and especially of PI, PE and PG was accompanied by acyl-CoA synthetase activation. The presence of two different enzymes, activating the saturated and unsaturated fatty acids is discussed.

Selective inhibition of lignoceroyl-CoA synthetase by adenosine 5'-alkylphosphates

Structural analogs of adenosine 5'-acylphosphates, which are intermediates of the reaction catalyzed by acyl-CoA synthetases, were synthesized by condensing primary alcohols with AMP to examine the inhibitory effects on the lignoceroyl-CoA and palmitoyl-CoA synthetase activities. Hexadecyl, octadecyl, eicosyl, docosyl and tetracosyl esters of AMP were remarkably potent inhibitors of the lignoceroyl-CoA formation. On the other hand, the eicosyl, docosyl or tetracosyl esters of AMP did not behave as significant inhibitors of the palmitoyl-CoA formation at the concentration at which the two other shorter chain analogs were effective. Namely, these longer alkyl esters of AMP have selective inhibitory effects on the lignoceroyl-CoA synthetase activity. The Ki value of adenosine 5'-tetracosylphosphate, the most potent inhibitor, was about one tenth lower than the Km value for the substrate lignoceric acid. Furthermore, the results support the notion that lignoceroyl-CoA synthetase is distinct from palmitoyl-CoA synthetase.

Activation of 3-methyl-branched fatty acids in rat liver

1. Subcellular fractionation of rat liver revealed that 3-methylmargaric acid, a monobranched phytanic acid analogue, can be activated by mitochondria, endoplasmic reticulum and peroxisomes. 2. Indirect data (effects of pyrophosphate and Triton X-100) suggested that the peroxisomal activation of 3-methylmargaric, 2-methylpalmitic and palmitic acid is catalyzed by different enzymes. 3. Despite many attempts, column chromatography of solubilized peroxisomal membrane proteins so far did not provide more conclusive data. On various matrices, lignoceroyl-CoA synthetase clearly eluted differently from the synthetases acting on 3-methylmargaric, 2-methylpalmitic and palmitic acid. The latter three however, tended to coelute together, although often not in an identical manner.

Activation of a peroxisome-proliferating catabolite of cholic acid to its CoA ester

We have shown that a microbial cholic acid catabolite (4R)-4-(2,3,4,6,6a beta,7,8,9,9a alpha,9b beta-decahydro-6a beta-methyl-3-oxo- 1H-cyclopenta[f]quinolin-7 beta-yl)valeric acid (DCQVA), is a potent peroxisome proliferator. In this paper a possible key stage in DCQVA metabolism, the activation of DCQVA to its CoA ester, has been investigated in rat liver microsomes and particulate fractions. The microsomal reaction was dependent on CoA, ATP, DCQVA (0.2-1 mM) and protein content. The reaction was decreased by storage at 4 degrees C, preincubation of microsomes at 37 degrees C for 5 min, or inclusion of Triton X-100 in the reaction mixture. Such treatments also enhanced generation of long-chain fatty acyl-CoAs, as determined by h.p.l.c. analysis. The same effect was caused by exposing the microsomes to phospholipase A2, suggesting that endogenous fatty acids may compete with DCQVA for esterification with CoA. Subcellular fractionation of rat liver demonstrated that the activity of DCQVA-CoA synthesis was localized predominantly in the microsomal fraction, in contrast to long-chain fatty acyl-CoA synthetase, which was distributed among all particulate fractions. Administration of clofibrate of rats did not affect the distribution of DCQVA-CoA synthesis activity. In contrast to a 2-fold induction of long-chain fatty acyl-CoA synthetase by clofibrate treatment, the activity of DCQVA-CoA synthesis in the microsomal fraction decreased by 80%. These results suggest that DCQVA is activated by an enzyme distinct from long-chain fatty acyl-CoA synthetase. The resulting perturbation of fatty acid metabolism may be involved in the mechanism whereby DCQVA causes peroxisome proliferation.

Purification of peroxisomes and subcellular distribution of enzyme activities for activation and oxidation of very-long-chain fatty acids in rat brain

Brain contains high amounts of very-long-chain (VLC) fatty acids (> C22). Since mitochondria from liver and skin fibroblasts lack lignoceroyl-CoA ligase, in liver and skin fibroblasts fatty acids are exclusively oxidized in peroxisomes. Findings by Poulos and associates [9] suggested that contrary to liver and cultured skin fibroblasts brain mitochondria contain lignoceroyl-CoA ligase and can oxidize lignoceric acid. The present study was undertaken to develop a procedure for the isolation of subcellular organelles of higher purity from brain and to get a better understanding of the subcellular localization of the oxidation of VLC fatty acids in brain. The enzyme activities for activation and oxidation of palmitic and lignoceric acids were determined in peroxisomes, mitochondria, microsomes and a myelin fraction from rat brain and peroxisomes, mitochondria and microsomes purified from rat liver. Like in liver, brain lignoceroyl-CoA ligase activity in microsomes and peroxisomes was approx. 9 times higher than in mitochondria. In addition to palmitoyl-CoA ligase the antibodies against palmitoyl-CoA ligase inhibited the residual mitochondrial lignoceroyl-CoA ligase activity, meaning that lignoceroyl-CoA ligase activity in mitochondria was derived from palmitoyl-CoA ligase. Accordingly, in peroxisomes lignoceric acid was oxidized at 7 times higher rate than in mitochondria. Mitochondria were able to oxidize lignoceric acid efficiently when supplemented with lignoceroyl-CoA ligase activity from microsomes or myelin. These results show that in brain lignoceric acid is oxidized in peroxisomes and that lignoceroyl-CoA ligase activity is localized in peroxisomes and microsomes, but not in mitochondria. Peroxisomes and microsomes contain both lignoceroyl-CoA and palmitoyl-CoA ligases. Similar to peroxisomes and microsomes, the antibodies against palmitoyl-CoA ligase inhibited only the palmitoyl-CoA ligase activity in myelin but not the lignoceroyl-CoA ligase activity. These results suggest that in addition to palmitoyl-CoA ligase, myelin also contains lignoceroyl-CoA ligase.

Studies on the effect of fenoprofen on the activation and oxidation of long chain and very long chain fatty acids in hepatocytes and subcellular fractions from rat liver

We studied the effect of fenoprofen on the activation of palmitic acid (C16:0), lignoceric acid (C24:0) and cerotic acid (C26:0) in microsomal and peroxisomal fractions from rat liver. Fenoprofen was found to inhibit the formation of palmitoyl-CoA in both microsomal and peroxisomal fractions whereas the formation of lignoceroyl-CoA and cerotoyl-CoA was not inhibited at all. In freshly isolated rat hepatocytes palmitic acid beta-oxidation was progressively inhibited at increasing concentrations of fenoprofen, most probably due to its inhibitory effect on palmitoyl-CoA synthetase activity. On the other hand, fenoprofen was also found to inhibit the beta-oxidation of lignoceric acid and cerotic acid in rat hepatocytes. It is shown that the acyl-CoA oxidase activity with lignoceroyl-CoA as substrate was inhibited by fenoprofen whereas the palmitoyl-CoA and pristanoyl-CoA oxidase activities were not inhibited by fenoprofen. This finding provides an explanation for the inhibitory effect of fenoprofen on lignocerate and cerotate beta-oxidation in hepatocytes.

Identification of phytanoyl-CoA ligase as a distinct acyl-CoA ligase in peroxisomes from cultured human skin fibroblasts

Phytanic acid accumulates in excessive amounts in Refsum disease, a rare neurological disorder, due to a defect in its alpha-oxidation enzyme system in peroxisomes. The activation of phytanic acid to phytanoyl-CoA by phytanoyl-CoA ligase is a prerequisite for its alpha-oxidation. The studies described in this manuscript report that phytanoyl-CoA ligase in peroxisomes is an enzyme distinct from the previously reported acyl-CoA ligases.

Decreased mitochondrial oxidation of fatty acids in pregnant mice: possible relevance to development of acute fatty liver of pregnancy

Severe impairment of the beta-oxidation of fatty acids, as a consequence of a single factor or a combination of different causes, leads to microvesicular steatosis of the liver. In an effort to understand the mechanism(s) leading to the development of acute fatty liver of pregnancy in some women, we determined the effects of pregnancy on the mitochondrial oxidation of fatty acids in mice. In vivo, the rate of oxidation of the whole fatty-acid chain length was determined by measuring the rate of exhalation of [14C]CO2 after intragastric administration of a tracer dose of [U-14C]palmitic acid. [14C]CO2 exhalation was not significantly decreased at 14 days of gestation, but it had declined by 40% at 18 days of gestation (i.e., 24 to 48 hr before delivery). The rate of first beta-oxidation cycle was assessed by measuring the rate of [14C]CO2 exhalation after administration of [1-14C]octanoic acid, [1-14C]butyric acid or [1-14C]palmitic acid. [14C]CO2 exhalation had declined by 60%, 46%, and 24% after administration of [1-14C]octanoic acid, [1-14C]butyric acid and [1-14C]palmitic acid, respectively, in 18-day-pregnant mice. Total hepatic lipids and triglycerides, expressed per gram of liver, remained unchanged in 18-day-pregnant mice. In vitro, the rate of mitochondrial beta-oxidation (expressed per milligram of protein) had decreased by 47% at 18 days' gestation with [U-14C]palmitic acid as substrate and by 33% with [1-14C]octanoic acid but remained unchanged with [1-14C]palmitic acid. The activity of the tricarboxylic acid cycle, assessed by the formation of [14C]CO2 from [1-14C]acetic acid, had decreased by 24%. We conclude that the mitochondrial oxidation of fatty acids decreased during late-term pregnancy in mice as a consequence of both decreased mitochondrial beta-oxidation of medium-chain fatty acids, and decreased activity of the tricarboxylic acid cycle. We suggest that this effect, in combination with other factors, may contribute to the development of fatty liver of pregnancy in some pregnant women.

Metabolic pathways in mammalian peroxisomes

This article summarizes our current knowledge of the metabolic pathways present in mammalian peroxisomes. Emphasis is placed on those aspects that are not covered by other articles in this issue: peroxisomal enzyme content and topology; the peroxisomal beta-oxidation system; substrates of peroxisomal beta-oxidation such as very-long-chain fatty acids, branched fatty acids, dicarboxylic fatty acids, prostaglandins and xenobiotics; the role of peroxisomes in the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen products such as hydrogen peroxide, superoxide anions and epoxides.

Subcellular distribution and characteristics of ciprofibroyl-CoA synthetase in rat liver. Its possible identity with long-chain acyl-CoA synthetase

The subcellular distribution and characteristics of ciprofibroyl-CoA synthetase were studied in rat liver and compared with those of long-chain acyl-CoA synthetase (palmitate as substrate) which, as already known, is distributed among mitochondria, microsomes and peroxisomes. Upon differential centrifugation, the subcellular distribution of ciprofibroyl-CoA synthetase followed closely that of palmitoyl-CoA synthetase and was specifically inactivated in the mitochondrial fraction by freezing and thawing, a behaviour already described for palmitoyl-CoA synthetase. Both enzyme activities were found to co-purify through several steps from rat liver microsomes. By using a partially purified enzyme, the activation of ciprofibrate to its acyl-CoA ester followed Michaelis-Menten kinetics with an apparent Km of 0.63 +/- 0.1 mM. Ciprofibroyl-CoA synthetase was competitively inhibited by 25 and 50 microM-palmitic acid. Higher concentrations of the fatty acid resulted in a mixed type of inhibition. Conversely, ciprofibrate up to 0.5 mM was found to inhibit competitively palmitoyl-CoA synthetase, whereas higher concentrations also resulted in a mixed inhibition. The highest activity of ciprofibroyl-CoA synthetase was found in fat and liver homogenates. The distribution of the enzyme in different rat tissues was similar to that of palmitoyl-CoA synthetase. The present results suggest that long-chain acyl-CoA synthetase and ciprofibroyl-CoA synthetase activities reside in identical or closely related proteins.

Autopsy findings in two siblings with infantile Refsum disease

Recognition of adrenal atrophy during a review of autopsy findings in two sisters who died at 8 months and 3 1/2 years prompted estimation of very long chain fatty acids, phytanic acid and pristanic acid on wet liver fixed in formalin for 12 years. These were shown to be markedly increased and defects in multiple peroxisomal functions and decrease in particulate catalase were shown in cultured fibroblasts, confirming an abnormality of peroxisomal biogenesis. The patients had presented with failure to thrive, recurrent diarrhoea and vomiting, poor mental development, retinal pigmentation, blindness and in the older patient deafness, with only mild dysmorphic features. Autopsy in the older patient showed adrenal atrophy, cirrhosis, and foamy histiocytes in multiple organs. The brain showed no demyelination, little cytoarchitectural abnormality, occasional perivascular histiocytes in the grey matter and meninges and prominent Purkinje cells in the molecular layer of the cerebellum. In the younger patient the changes were very subtle in spite of the marked clinical similarity. Despite the young age at death the clinicopathological features are most suggestive of infantile Refsum disease. In many situations anatomical pathology can be very useful in the recognition and study of peroxisomal disorders.